It is conventional to provide a magnetic metal detector adjacent the crop feed path of a harvester for the purpose of detecting the presence of ferrous metal objects picked up from a field with the crop material. Upon detection of a metal object in the crop feed path an output signal is produced to stop the crop feed mechanism before the metal object is fed into a crop cutter mechanism where it could cause considerable damage to the cutter knives and shear bar. Exemplary harvesters with metal detectors are described in U.S. Pat. Nos. 3,757,501, 3,889,249, 3,972,156 and 4,433,528.
The metal detectors must, of necessity, be located close to the crop feed path so that metal objects passing through the feed path may also pass through the magnetic detection field of the detector. As a result, electrical "noise" is introduced into the metal detector by moving metal parts of the harvester which disturb the detection field thereby inducing a noise emf component in the detection coils. U.S. Pat. No. 4,854,113 provides a system wherein much of the cyclically recurring noise produced in a metal detector is cancelled. However, this system does not eliminate non-cyclic noise.
The noise component introduced into a metal detector creates a problem in that it adds to and subtracts from, the detection component in the detection circuits. Generally speaking, the output signal from the detection circuits is compared with threshold values chosen to represent the minimum positive or negative magnitude of the detector output signal when a metal object is detected. If one of these threshold levels is exceeded then the crop feed mechanism is stopped.
Because of the noise component in the detector output signals, the detection threshold levels must be set higher than if the noise component were not present. This effectively reduces the sensitivity of the system and it may not detect metal objects it would otherwise detect. On the other hand, if the threshold levels are set too low, the noise component may cause a false indication that a metal object has been detected.
The problem is further complicated in that the noise component tends to increase as the moving machine parts which cause the noise age and wear. Thus, adaptive thresholding was developed to automatically vary the detection threshold as the noise component in the output signal from the detection circuits increases. U.S. Pat. Nos. 3,757,701 and 3,889,249 disclose a metal detector system with adaptive thresholding for compensating for variations in the noise component. However, in the system disclosed in these patents electronic circuits are required to develop the threshold control signal and this adds to the cost of the system. Furthermore, this prior art system does not provide for adjusting both the positive and negative thresholds. Still further, the prior art system must begin a new development of the threshold levels each time the system is turned on. Thus, at start-up the system may produce a false indication of metal object detection.